Fig. 1: Phosphate (P_i) acquisition by marine bacteria.

a In a model cell labelled with *P_i the *P_i-tracer can be (i) extracellular, i.e., adsorbed onto the cell surface; (ii) accumulated in the periplasm; (iii) bound to the PstS subunit in the periplasm; (iv) in a labile form in the cytoplasm, e.g., soluble P_i, nucleotides, sugar phosphates, small molecules of RNA; and (v) in a non-labile form of assimilated P, e.g., DNA, ribosomal RNA, polyphosphates, phospholipids. a† Current understanding—parallel processes of passive adsorption of P_i to the bacterial cell surface composed of polymers of the outer membrane lipopolysaccharide (LPS) and overlaid (in some bacteria) by the proteinaceous surface layer (S-layer) and P_i diffusion across the outer membrane through porins into the periplasm. Diffusion is coupled directly to P_i transport across the cytoplasmic membrane by ABC-type transporters (PstCAB) using the periplasmic P_i-binding protein (PstS). a‡ The proposed model—mass transfer of P_i anions across the outer membrane through porins, their buffering by cations of the membrane potential in the periplasm and import of P_i from the buffered stock through the cytoplasmic membrane by PstCAB using P_i-PstS. Isotonic or slightly hypertonic osmotic pressure is maintained in the periplasm by the buffered P_i salt. b–e Removal of different phosphorus pools by washing or fixation of a model *P_i-labelled Synechococcus cell. b Treatment with surfactants, hydrolytic enzymes and amended ASW removes the extracellular P_i adsorbed to cell surface constituents. c A short wash with hypotonic solution, e.g., deionized water (DW), dissolves the extracellularly adsorbed P_i and, by causing osmotic shock, releases the periplasmic contents of a cell, i.e., the accumulated and PstS-bound P_i. d Fixation of cells with paraformaldehyde (PFA) compromises the outer and inner membranes, releasing labile periplasmic and intracellular P_i, but crosslinks cellular proteins, immobilizing the primary phosphorus-containing macromolecules including DNA, rRNA and P_i-carrying PstS subunits. e Fixation of cells with trichloroacetic acid (TCA) disrupts membranes and precipitates cellular macromolecules (proteins, nucleic acids, polyphosphates and polysaccharides), thereby releasing labile P_i but immobilizing most of the assimilated P_i.